Wet-magnetic separator



Aug. 9, 1955 B. c. SCHMID 2,714,960

WET-MAGNETIC SEPARATOR Filed Oct. 7, 1952 3 Sheets-Sheet l INVENTOP BYRON Cjcnmlu ATTORNEY 5 SheetsSheet 2 Filed Oct. 7, 1952 li l INVENTOR BYRON 6. 5c HM/U Aug. 9, 1955 B. c. SCHMID WET-MAGNETIC SEPARATOR 3 Sheets-Sheet 3 H68 70 Filed Oct. 7, 1952 INVENTOR BY Bree/v C. 501mm ATTORNEY United States Patent 0 WET-MAGNETIC SEPARATOR Byron C. Schmid, Washington, D. C., assignor to the United States of America as represented by the Solicitor of the Department of the Interior Application October 7, 1952, Serial No. 313,617 8 Claims. (Cl. 209-227) (Granted under Title 35, U. S. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government of the United States for governmental purposes without the payment to me of any royalty thereon in accordance with the provisions of the Act of April 30, 1928 (Ch. 460, 45 Stat. L. 467).

This invention relates to a magnetic separating apparatus, and more particularly, to an apparatus of this type for separating magnetic solid particles from non-magnetic solid particles carried by a fluid stream.

The magnetic separator of this invention is particularly useful in mineral separation processes on a laboratorvor pilot plant scale but may be used in any process involving separation of magnetic material from non-magnetic material. Laboratory magnetic separators heretofore available provide a relatively clean separation but can be used only on very small samples and their operation is tedious and time consuming. Larger magnetic separators of the commercial type have a far larger capacity but provide less eifective separation. The device of th1s invention may be used either as a laboratory separator or as a larger unit for industrial applications. For large scale operations of the latter type, the separator may be installed in batteries in a manner akin to the use of multiple units of Humpherys spiral separator.

An object of this invention is to provide a magnetic separator which is easy to operate and will yield a clean magnetic concentrate.

Another object of this invention is to provide a magnetic separator which will give a quick and effective separation of large samples of material with production of a clean magnetic concentrate.

A further object of this invention is to provide a continuous wet-magnetic separator which will give a clean magnetic concentrate with the use of a minimum of washing liquid.

A still further object of this invention is to provide a continuous wet-magnetic separator which will give a clean magnetic concentrate and which is readily adjustable for dilferent feed material.

Other related objects and advantages of the invention will be apparent as the ensuing description proceeds.

The magnetic separator of this invention, in general, comprises a plurality of pairs of magnetic pole pieces supported for rotation about a fixed central axis, means for rotating the pole pieces about the axis, a tube forming an incomplete circle supported in the air gap' between the magnetic pole pieces, feed means intermediate the ends of the tube for introducing the material to be separated, discharge means for magnetic particles at one end of the tube, discharge means for non'magnetic particles at the other end of the tube, means for introducing washing liquid into the tube adjacent the discharge end for magnetic particles, and means for periodically interrupting the magnetic field in the air gap between the pole pieces.

The axis of rotation of the pairs of magnetic pole pieces preferably is vertical, but it may be inclined toward the horizontal or may be horizontal without departing from the spirit and scope of this invention. Where the axis 2,714,960 Patented Aug. 9, 1955 of rotation is vertical a flux divertor is employed at the discharge end for magnetic particles in order to facilitate their discharge from the tube. Where the axis of rotation of the pole pieces is tipped toward the discharge end for magnetic particles discharge may be accomplished without a flux divertor.

The magnetic flux between each pair of pole pieces may be produced by permanent magnets or by electromagnets, but it is preferred to use the latter since interruption of the magnetic flux is more readily accomplished when magnets of this type are employed.

Each pair of pole pieces is so oriented that the average direction of the flux of each pair is approximately parallel to the axis about which it revolves. The size and shape of each pair of pole pieces and the distance between their faces may be varied within the limits of good design prac tice. The pole pieces of a given pair may be of different shapes, but it is preferred that each pair of pole pieces be identical in size and shape to every other pair. The distance between the pole faces of each pair is normally the same. The centers of the air gaps for all pairs of poles is on the circumference of a circle whose center lies on and whose plane is perpendicular to the axis of rotation. It is preferred to have the centers of these air gaps symmetrically spaced on the circumference of this circle. The number of pairs of pole pieces may be varied as desired depending on the size of the apparatus. For a unit having a diameter of about 20 inches, four to six pairs are satisfactory.

The circular tube or channel within the air gap may be imagined as being made by bending a length of tubing (of non-magnetic material) closed at each end into an almost complete circle whose average diameter is equal to the diameter of the circle on which the centers of the air gaps lie. The cross-section of the tubing may be of any desired shape. Tubes with circular or with rectangular cross-sections are satisfactory. The most efficient cross-section is believed to be a rectangle enlarged by segments of circles on the sides parallel to the axis of rotation of the pairs of magnet poles. The two over-all dimensions of this cross-section are limited by the distancebetween the pole faces in one direction and by the diminution in intensity of the strong flux field in the other.

Discharge openings are provided in the vicinity of each closed end of the tube for the attachment of lead off tubes. These tubes take an essentially downward course. One is the discharge tube for magnetic material and the other is the discharge tube for non-magnetic material and wash liquid. The function of the tube depends on the direction or rotation of the magnetic system. The magnetic discharge tube may be connected to a trap' through which magnetic material may be discharged while a hydrostatic head is maintained within the system. The non-magnetic material discharges into a sealed container provided with an overflow higher than the top of the separating tube but lower than the top of the feed hopper. Thus the separating tube is kept filled with liquid.

The feed inlet is provided in the circular tube at a point intermediate the two discharge outlets. The point at which this inlet is located is somewhat dependent upon the type of material treated. It is preferred to locate this inlet near the discharge end for non-magnetic material but a point about midway between the two ends is satisfactory. The inlet for wash liquid is provided at a point adjacent the discharge for magnetic material. Alternatively, wash liquid may be introduced through the discharge tube for the magneticmaterial counter-current to the falling magnetic particles.

With the foregoing considerations in mind, the invention will be made clear by the following detailed description of several forms of the apparatus taken in conjunction with the accompanying drawings, in which:

Fig. l is an elevational view, partially in section, of one embodiment of the magnetic separator of this invention;

Fig. 2 is a sectional view of the apparatus along the lines 22 of Fig. 1 looking in the direction of the arrows;

Fig. 3 is an enlarged sectional view along the lines 33 of Fig. 2 showing in elevation the flux divertor at the discharge end of the magnetic separator;

Fig. 4 is an enlarged detail side view, partially broken away, showing in elevation the tripping switch of the magnetic separator of Figs. 1 and 2;

Fig. 5 is a wiring diagram showing the electrical circuit used in the magnetic separator of Figs. 1 and 2;

Fig. 6 is an elevational view, partially in section, of a modification of the apparatus shown in Figs. 1 and 2;

Fig. 7 is a sectional view taken along the lines 7-7 of Fig. 6 looking in the direction of the arrows;

Fig. 8 is an enlarged detail view in section showing a modification of the upper pole pieces of the magnetic separator of this invention.

For a practical embodiment of the invention and referringnow to the drawings, a suitable basic framework is provided for supporting the rotatable magnets and stationary tubing. As shown in Fig. 1, this may take the form of a vertical shaft or axle 12 of non-magnetic stainless steel, brass or other suitable non-magnetic material, welded or otherwise fixed to a base 14 also of non-magnetic material. The base 14 conveniently may be of tripod or other suitable configuration. A brass collar 15 rests on the base 14 and acts as a bearing surface for the rotatable parts of the apparatus.

A cylinder or core 16 made of hot-rolled steel or other magnetic material is rotatably mounted about the shaft 12 and upon the bearing 15. This core is provided with a plurality of lower arms 17 and corresponding upper arms 18 made of similar material to the core 16 and welded or otherwise fixed thereto. Six pairs of arms are shown in this embodiment of the invention (Fig. 2) but 21 fewer or greater number of arms uniformly spaced around the core 16 may be employed. Each of the lower arms 17 are provided at their outer extremities with pole pieces 19 which are likewise of similar material to core 16 and arms 17. Upper arms 18 are likewise provided with pole pieces 20. The pole pieces may be welded or bolted to their respective arms, or if desired, the core, pole pieces, and arms may be cast in a single piece. The upper face of the lower pole pieces 19 are preferably serrated as shown at 21.

A magnetizing coil 22 is provided around the core 16 and is insulated therefrom by means of a liner 23 of nonconducting material. The coil 22 is connected to a source of electrical energy through lines 24, slip rings 25, brushes 26, lines 27 and switch 28. A condenser 29 is connected across the switch 28 by means of lines 30 and 31. Details of the electrical circuit and of the switch will be described below in connection with Figs. 4 and 5. The slip rings 25 are supported by a collar 32 of Bakelite or other insulating material mounted upon flange 33 which is fixed to core 16 and is adapted to rotate therewith around the shaft 12. The brushes 26 are supported by a bracket 34 likewise of Bakelite or other insulating material fixed to a collar 35 secured to shaft 12.

An annularly shaped pulley wheel 36 of Masonite or other suitable non-magnetic material is fixed to the bot tom of the arms 17 as by bolts 37. A belt 38 connects the pulley to a suitable source of power, not shown. In operation, the magnetic system is rotated by means of the pulley and belt.

The tube unit, as shown in Figs. 1 and 2 comprises a circular tube 40 which may be formed of a top piece bent to provide top and side walls and bolted to bottom piece 41 and to supporting plate 42. The supporting plate 42 is annular in shape and constructed of aluminum or other non-magnetic material. It is supported in the air-gap between the upper and lower pole pieces by means of vertical uprights 43. The tube 40 may be constructed of Plexiglas, or other form of glass in order that the separating action may be observed, or may be made of any other suitable non-magnetic metal.

As shown in Fig. 2, the tube 41) forms a substantially complete circle in the air-gap. The pole-pieces in the apparatus shown are designed to rotate in a counterclockwise direction looking down at Fig. 2. A discharge tube 44 is provided at one end for non-magnetic particles and a discharge tube 45 is provided at the other end for magnetic particles. A feed inlet 46 is provided at a point near the discharge end for the non-magnetic particles and is connected to a feed hopper 47. An inlet 48 for wash liquid is provided near the discharge end for the magnetic particles.

The tube 4% is curved outwardly from the air-gap near the discharge end for magnetic particles as shown at 49. The lines of magnetic flux are diverted at this point to permit discharge of the magnetic particles by gravity through outlet tube 45. A suitable flux divertor, as shown in Fig. 3, comprises a piece of magnetizable material 50, such as iron, supported over the tube 49 above the discharge opening by a bracket 51 of non-magnetic material secured to the annular plate 42. A second piece of magnetizable material 52 is attached to the piece and extends horizontally and circularly parallels the path of the rotating pole pieces through its entire length. The inner face of the piece 52 may be curved to correspond to the are described by the outside of the rotating pole pieces 20 with sufficient clearance allowed to avoid being struck by these pole pieces. In operation, magnetism is temporarily induced in the pieces 50 and 52 by each passing pair of poles. The temporarily induced magnetism causes particles of magnetic material to be carried over the discharge opening where they are dropped when the magnetic attraction decreases due to the passage of the pole pieces away from the pieces 50 and 52.

The current through the magnetizing coil 22 is periodically broken by tripping means attached to the upper pole pieces 20 for operating switch 28. The tripping means as shown in detail in Fig. 4 may be in the form of a strip 54 of resilient material attached to the pole pieces 20 by bolts or other means at 55 and provided with a cross-arm 56 for contacting a projection 57 on switch panel 58 as the pole piece rotates. The switch tripping means may be provided on any desired number of pole pieces but preferably are affixed to alternate pole pieces. The switch comprises a switch blade 59 pivotally fixed to the panel 58 at point 60 and provided with an extending camshaped arm 61, and a stationary contact plate 62. The switch panel is supported by arm 63 fixed to the shaft 12 as shown in Fig. 1.

The circuit from a source of direct current through the switch 28 and magnetizing coil 22 is shown in Fig. 5.

In operation, as a magnetic pole piece with tripping means attached rotates, the cross piece 56 first comes in contact with projection 57 attached to the panel 58. Continued rotation causes the strip 54 to be bent back and down until the crosspiece 56 passes underneath the projection 57. At this instance the resilience of the strip 54 snaps the cross piece forward and the cross piece 56 hits the movable switch blade 59 a hammer-like blow causing it to move upwardly breaking the electrical circuit through the switch and thus interrupting the current through magnetizing coil 22. Since condenser 29 is connected in parallel with the switch, the current decays to zero in an oscillating manner. This has a demagnetizing efiect on any magnetic material in the air gaps. Further rotation of the pole piece causes the cam 61 to ride smoothly backward on the cross piece 56 until electrical contact is made. The arm 63 supporting the switch may be adjusted to place the switch at any point around the circular tube at which breakup of the magnetized particles is desired in order to enable them to be thoroughly washed by the liquid flowing in countercurrent relation to them. Preferably, the switch is placed in the inlet for the wash liquid.

Another means for permitting break-up of magnetized particles in the tube 40 may be provided as shown in Fig. 8. In accordance with this modification of the invention the top pole pieces are made to rotate with respect to their supporting arms when suitably tripped. Pole piece a is attached to arm 18a by means of shaft 64 of magnetic material secured in an opening in arm 18a by set screw 65. A hearing 66 of brass or other suitable non-magnetizable material is provided between shaft 64 and pole piece 20a. A plate 67 of similar material to bearing 66 keeps the pole piece in place on the shaft. A tripping rod 68 adapted to contact a fixed overhead projection (not shown) is connected to the top of pole piece 20a as by means of brass plate 69 secured by screws 70. The tripping means is preferably placed on alternate pole pieces. In operation, tilting of the pole piece 20a is accomplished when rod 68 strikes the overhead projection. This increases the air gap between the upper and lower pole pieces and causes the magnetic field intensity to decrease to an extent that releases the bunched-up collection of magnetic material, thereby dispersing it in the waste liquid. The pole piece resumes its normal position after tripping by means of the force of its own weight and magnetic force. The released magnetic particles are picked up by the following pole which carries them further around the cycle.

In the modification of the apparatus shown in Figs. 6 and 7, the lower pole pieces are given a planetary motion in order to furnish additional agitation of the magnetic particles in the tube 40. This is accomplished by means of a stationary gear 71 fixed to shaft 12 below the bearing 15 on which the magnetic assembly rotates. The gear 71 is spaced from the base 14 by collar or ring 72. The lower pole pieces 1% are provided at their lower ends with cylindrical shafts 73 which are mounted for rotation in cylindrical openings provided in the lower arm members 17a and underlying drive pulley 36. A collar 74 of brass or other suitable material provides a hearing surface and facilitates the rotation of the pole pieces. Planetary gears 75 are fixed to the shafts 73 of the lower pole pieces 19a as by means of a threaded pin 76 and cause the pole pieces to rotate with a planetary motion as the assembly rotates around the stationary gear 71. The upper poles may be provided with either tripping means for breaking the current to the magnetizing coil, or may be designed to tilt in accordance with the modification shown in Fig. 8. For purposes of illustration, they are shown in Fig. 6 provided with the tripping means of Fig. 8.

In order that the operation of the device be fully understood, atypical operating cycle with the form of the device as shown in Figs. l-5 will now be described. The wash water is turned on and enters the tube through inlet 48 thereby filling the system. The wash water flows in a clockwise direction looking down at Fig. 2 due to the hydrostatic head maintained on the system through a trap (not shown) following the discharge outlet for the magnetic particles. The wash Water flows out through discharge 44 for the non-magnetic particles.

The current to the magnetizing coil 22 is now turned on, and the switch 28 being in closed position, the coil is energized and causes magnetic flux to flow through the pole pieces.

The power which causes the pulley wheel 36 to rotate is then turned on causing the magnetic assembly to rotate in a counterclockwise direction. A rotational speed of about 3 r. p. m. is satisfactory for units of twenty-inch diameter, but the speed may be adjusted as desired depending upon the size of the unit and the composition and volume of the feed. Conditions affecting the separation may be varied by varying the volume and velocity of the wash water, the feed rate, the speed of rotation of the pole pieces, and the field intensity.

The hopper 47 is now fed with a slurry of ground and position to be tripped near screened material to be separated. This material may be, for example, a sinter of titaniferous magnetite containing particles of metallic iron. Each revolving pair of poles carries a quantity of magnetic material from the feed inlet 46 toward the magnetic discharge end 49 countercurrent to the flow of wash Water. The switch 28 in the magnetizing circuit is periodically tripped by the tripping means on the alternate pole pieces thereby causing the particles to become demagnetized, dispersed, and thoroughly contacted with the wash water before they are again picked up when the switch 28 is again closed. The length of the cam 61 governs the time period during which the switch contact is broken. The magnetic material is picked up and dropped a number of times and the pole pieces may make several complete revolutions before the magnetic material finally reaches the discharge end. At this point, it is pulled by the flux divertor 50 over the discharge opening and then dropped through the discharge tube into the trap (not shown). The wash water, moving clockwise, continuously carries non-magnetic particles out of discharge tube 44. The operation may be continued as long as desired.

The modified form of the apparatus shown in Figs. 6, 7, and 8 is operated in similar manner. In this form of the apparatus, however, the particles of material are agitated and dispersed by means of the rotating action of the lower pole pieces 19a, and by the tilting of the upper pole pieces 20a.

It will be appreciated from a reading of the foregoing specification that the invention herein described is susceptible of various changes and modifications without departing from the spirit and scope thereof.

What is claimed is:

1. A magnetic separator comprising a plurality of pairs of magnetic pole pieces supported for rotation about a fixed substantially vertical central axis, the poles of each of said pairs of pole pieces being spaced to form an air gap therebetween, means for rotating the pole pieces about the axis, means for providing a magnetic field between the poles of said pairs of pole pieces, a tube of non-magnetic material forming an incomplete circle supported in the air gap between the magnetic pole pieces, feed means intermediate the ends of the tube for introducing material to be separated, discharge means for magnetic particles at one end of the tube, discharge means for nonmagnetic particles at the other end of the tube, means for introducing washing liquid into the tube adjacent the discharge end for magnetic particles, and means responsive to movement of said pole pieces through a selected position for periodically interrupting the magnetic field in the air gap between the pole pieces.

2. A magnetic separator comprising a plurality of pairs of magnetic pole pieces supported for rotation about a fixed substantially vertical central axis, the poles of each of said pairs of pole pieces being spaced to form an air gap therebetween, means for rotating said pole pieces about the axis, an electromagnet for inducing magnetism in said pole pieces, an electrical circuit connecting said electromagnet to a source of direct current, a switch in said electrical circuit, means for rotating the pole pieces about said axis, a tube of non-magnetic material forming an incomplete circle supported in the air gap between the magnetic pole pieces, feed means intermediate the end of the tube for introducing material to be separated, discharge means for magnetic particles at one end of the tube, discharge means for non-magnetic particles at the other end of the tube, means for introducing washing liquid into the tube adjacent the discharge end for magnetic particles, and means responsive to movement of said pole pieces through a selected position for periodically tripping the switch in the electrical circuit to the electromagnet.

3. A magnetic separator comprising a plurality of pairs of magnetic pole pieces supported for rotation about a fixed substantially vertical central axis, the poles of each of said pairs of pole pieces being spaced to form an air gap therebetween, means for rotating said pole pieces about the axis, an electromagnet for inducing magnetism in said pole pieces, an electrical circuit connecting said electromagnet to a source of direct current, a switch in said electrical circuit, a condenser in said electrical circuit connected across said switch, means for rotating the pole pieces about the axis, a tube of non-magnetic material forming an incomplete circle supported in the air gap between the magnetic pole pieces, feed means intermediate the ends of the tube for introducing material to be separated, discharge means for magnetic particles at one end of the tube, discharge means for nonmagnetic particles at the other end of the tube, means for introducing washing liquid into the tube adjacent the discharge end for magnetic particles, and tripping means mounted on at least one of said pole pieces for tripping said switch in the electrical circuit to the electromagnet at a selected point in the path of rotation of said pole pieces.

4. A magnetic separator comprising a plurality of pairs of magnetic pole pieces supported for rotation about a fixed substantially vertical central axis, the poles of each of said pairs of pole pieces being spaced to form an air gap therebetween, means for rotating the pole pieces about said axis, an electromagnet for inducing magnetism in said pole pieces, an electrical circuit connecting said electromagnet to a source of direct current, a switch in said electrical circuit having a cam-shaped arm extending above the path of rotation of said pole pieces, tripping means mounted on at least one of said pole pieces adapted to engage said switch arm and ride along the cam surface as the pole piece rotates thereby periodically breaking the electrical circuit to said electromagnet, a tube of non-magnetic material forming an incomplete circle supported in the air gap between the magnetic pole pieces, feed means intermediate the ends of the tube for introducing material to be separated, discharge means for magnetic particles at one end of the tube, discharge means for non-magnetic particles at the other end of the tube, and means for introducing wash liquid into the tube adjacent the discharge end for magnetic particles.

5. A magnetic separator comprising a plurality of pairs of magnetic pole pieces having a common core supported for rotation about a fixed central vertical axis, the poles of each of said pairs of pole pieces being spaced to form an air gap therebetween, means for rotating said core and pole pieces about the axis, means for providing a flow of magnetic flux between said pole pieces a tube of non-magnetic material forming an incomplete circle supported in the air gap between the magnetic pole pieces and diverted from the air gap at one end, feed means intermediate the ends of the tube for introducing material to be separated, a discharge outlet for magnetic particles at the diverted end of said tube, a discharge outlet for non-magnetic particles at the other end of said tube, means for introducing washing liquid into the tube adjacent the discharge end for magnetic particles, means for periodically interrupting the magnetic field in the air gap between the pole pieces, and flux-diverting means adjacent the discharge end of said tube for magnetic particles adapted to be magnetized by the rotating pole pieces to divert magnetic particles to the discharge outlet.

6. A magnetic separator comprising a plurality of pairs of magnetic pole pieces supported for rotation about a fixed vertical axis, each pair of pole pieces includnig an upper pole and a lower pole separated by an air gap, means for rotating said pairs of pole pieces about said vertical axis, planetary means for rotating said lower pole pieces with a planetary motion, means for providing a magnetic field between the poles of said pole pieces, a tube of non-magnetic material forming an incomplete circle supported in the air gap between the pole pieces, feed means intermediate the ends of said tube for introducing material to be separated, a discharge outlet for magnetic particles at the other end of said tube, and means for introducing wash liquid into said tube adjacent the discharge end for magnetic particles.

7. A magnetic separator comprisinga plurality of pairs of magnetic pole pieces supported for rotation about a fixed vertical axis, each pair of pole pieces including an upper pole and a lower pole separated by an air gap, means for rotating said pairs of pole pieces about said vertical axis, planetary means for rotating said lower pole pieces with a planetary motion, means for providing a magnetic field between the poles of said pole pieces means responsive to movement of said pole pieces through a selected position for periodically interrupting the magnetic field in the air gap between the pole pieces, a tube of non-magnetic material forming an incomplete circle supported in the air gap between the pole pieces, feed means intermediate the ends of said tube for introducing material to be separated, a discharge outlet for magnetic particles at one end of said tube, a discharge outlet for non-magnetic particles at the other end of said tube, and means for introducing wash liquid into said tube adjacent the discharge end for magnetic particles.

8. A magnetic separator comprising a plurality of pairs of magnetic pole pieces supported for rotation about a fixed vertical axis, means for providing a magnetic field between the poles of said pole pieces each pair of pole pieces including an upper pole piece and a lower pole piece separated by an air gap, means for rotating said pairs of pole pieces about said vertical axis, at least one of said pole pieces being rotatable about a horizontal axis, means for periodically tipping said pole piece about said horizontal axis as the pairs of pole pieces rotate about said vertical axis, a tube of non-magnetic material forming an incomplete circle supported in the air gap between the pairs of pole pieces, feed means intermediate the ends of the tube for introducing material to be separated, discharge means for magnetic particles at one end of said tube, discharge means for non-magnetic particles at the other end of said tube and means for introducing washing liquid adjacent the discharge end for magnetic particles.

References Cited in the file of this patent UNITED STATES PATENTS 402,684 Maxim May 7, 1889 466,514 Reed Jan. 5, 1892 466,515 Reed Jan. 5, 1892 731,443 Eriksson June 23, 1903 2,282,510 BeGor May 12, 1942 

1. A MAGNETIC SEPARATOR COMPRISING A PLURALITY OF PAIRS OF MAGNETIC POLE PIECES SUPPORTED FOR ROTATION ABOUT A FIXED SUBSTANTIALLY VERTICAL CENTRAL AXIS, THE POLES OF EACH OF SAID PAIRS OF POLE PIECES BEING SPACED TO FORM AN AIR GAP THEREBETWEEN, MEANS FOR ROTATING THE POLE PIECES ABOUT THE AXIS, MEANS FOR PROVIDING A MAGNETIC FIELD BETWEEN THE POLES OF SAID PAIRS OF POLE PIECES, A TUBE OF NON-MAGNETIC MATERIAL FORMING AN INCOMPLETE CIRCLE SUPPORTED IN THE AIR GAP BETWEEN THE MAGNETIC POLE PIECES, FEED MEANS INTERMEDIATE THE ENDS OF THE TUBE FOR INTRODUCING MATERIAL TO BE SEPARATED, DISCHARGE MEANS FOR MAGNETIC PARTICLES AT ONE END OF THE TUBE, DISCHARGE MEANS FOR NONMAGNETIC PARTICLES AT THE OTHER END OF THE TUBE, MEANS FOR INTRODUCING WASHING LIQUID INTO THE TUBE ADJACENT THE DISCHARGE END FOR MAGNETIC PARTICLES, AND MEANS RESPONSIVE TO MOVEMENT OF SAID POLES PIECES THROUGH A SELECTED POSITION FOR PERIODICALLY INTERRUPTING THE MAGNETIC FIELD IN THE AIR GAP BETWEEN THE POLES PIECES. 